This invention relates to optical waveguides, and more particularly to an apparatus and method for forming circular cross-section channel waveguides and monolithic integrated lenses.
An optical fiber typically has a circular cross-section shape. Optical fibers are sometimes coupled by channel waveguides. An end of an optical fiber maY be bonded or otherwise secured to an end of a channel waveguide. Typically, at the other end of the channel, another optical fiber end is secured to receive the optical signal as it travels through the channel. The geometric cross-section shape of a channel waveguide has been square or rectangular. With a light signal traveling from a circular cross-section fiber into a rectangular cross-section channel, there will be a degree of signal loss due to the geometric mismatch.
Channel waveguides have previously been made by deposition on a silicon substrate followed by an etching process. Lenses used to assist coupling between optical elements have usually been hybrid bonded to the substrate. Aligning and bonding discrete lenses is difficult and time-consuming. With the present invention, monolithic lenses are formed on the same substrate with aligned channel waveguides. The formation of circular cross section channel waveguides and spherical lenses on the same substrate are accomplished simultaneously which eliminates previously known aligning and bonding difficulties.
The present invention offers advantages over previously known devices and methods With the present invention substantially circular cross-section channel waveguides are formed. Having the channel waveguides in closer geometric agreement with the optical fibers will result in much lower coupling losses. The method of the present invention may also be used to form monolithic integrated lenses to be used with the channel waveguides to improve coupling efficiency between fibers, waveguides, laser diodes, detectors, and other circuit devices.
Two different methods are revealed by the present invention to make circular cross-section channel waveguides. Both methods are founded on the principle that the surface tension in the channel waveguide will change the shape of the waveguide cross-section from rectangular to circular as the waveguide is heated above its softening temperature. In the first method, square cross-section channel waveguides are formed by planar deposition and etching. Then a selective etchant etches the cladding material to reshape the waveguide. Heat is then applied in a predetermined fashion depending upon the glass composition of the waveguide. A circular shape is obtained due to the surface tension resulting in the glass.
In the second method, a planar waveguide is etched into a vertically rectangular cross-section channel having a cross-section area equal to the final desired circular cross-section area. Heat is applied to allow surface tension in the channel to function As the heat is applied, the core layer and cladding layer will respond differently, resulting in surface tension forcing the core to form a circular cross-section. With either the first or second method, a thin layer of material may be deposited over the core to form an over-cladding layer after the circular cross-section is obtained.
The following references are cited to show the state of the art: M. Kawachi, M. Yasu, and T. Edahiro, "Fabrication of SiO.sub.2 -TiO.sub.2 Glass Planar Optical Waveguides by Flame Hydrolysis Deposition", Electron Lett. Vol. 19, pp. 583, 584 (1983); T. Miyashita, M. Kawachi, and M. Kobayashi, "Silicon Based Planar Waveguides for Passive Components", Technical Digest, Optical Fiber Communication Conference (1988), Paper THJ 3; T. Miyashita, S. Sumida, S. Sakaguchi, "Integrated Optical Devices Based on Silica Waveguide Technologies", SPIE Vol. 993 Integrated Optical Circuit Engineering VI, pp. 288-291 (1988); S. Valette, et al., "Si-Based Integrated Optics Technologies", Solid State Technology, pp. 69-75, (Feb. 1989); Y. Yamada, M. Kawachi, M. Yasu, and M. Kobayashi, "High-Silica Multi-Mode Channel Waveguide Structure for Minimizing Fiber Waveguide Fiber Coupling Loss" , J. Lightwave Technol. Vol. Lt-4, 277 (1986); M. Kawachi, T. Edahiro and H. Toba, "Microlens Formation on VAD Single-Mode Fiber Ends" , Electron Lett. Vol. 18, 71 (1982); J. F. Oliver, C. Huh, and S. G. Mason, "Resistance to Spreading of Liquids by Sharp Edges" , Journal Colloide and Interface Science, Vol. 59, No. 3, p. 568 (1977); D. Alles, "Trends in Laser Packaging" Symposium of 40th Electronic Components and Technology Conference, p. 185 (1990); L. A. Reith, J. W. Mann, N. Andreadakis, G. R. Lalk, and C. E. Zah, "Single-Mode Fiber Packaging for Semi-Conductor Optical Devices" Symposium of 40th Electronic Components and Technology Conference, p. 193 ( 1990); R. Colclaser, "Micro-electronics: Processing and Device Design" , published by John Wiley & Sons, Inc., pp. 44, 45 (1980); A. S. Tenney, M. Ghezzo, "Etch Rates of Doped Oxides in Solutions of Buffered HF", Electrochem SOC: Solid State Science and Technology, Vol. 120, No. 8, p. 1091 (1973).
The foregoing and other objects and advantages will become more apparent when viewed in light of the accompanying drawings and the following description: